Find the first Saturday in your data, then assign a week ID to all dates in your data set based on that : library(lubridate) # for the wday() and ymd() functions daily_FWIH$Date <- ymd(daily_FWIH$Date) saturdays <- daily_FWIH[wday(daily_FWIH$Date) == 7, ] # filter for Saturdays startDate <- min(saturdays$Date) # select first...